Well this is more of an opinion, I will be starting a blog in the near future.
Well first of PHP is so quick to install with servers as the Apache HTTP server, there are a few configuration changes to apcahe HTTP httpd.config and the PHP.ini file but once that's solved it's done and ready to deploy an application you can think of.
Also you can use quick set up solutions as in wampserver or lampserver depending on what operating system you are running on.
I am still yet to test the scalability of PHP applications but will do so as soon as I can drive enough traffic to a web application.
By the way this is not spam.
I also like PHP as it is still a language in it's infancy, maybe it's loosely typed and it's not a true oop language but it does get things done quickly and with great productivity, with all the beautiful frameworks for PHP as in cakePHP, YII, codeIgniter and the Zend framework It just makes it such a great tool for conceptualization and prototyping of applications for the web and mobile.
Of course we could argue with languages as in asp.net or jsp well those are great languages for web development with the strict type and true oop of Java and C# but I do think they should be used for much large applications then your basic sites and apps like I would use jsp for a banking app or payment system and if it was a microsoft project then of course asp.net but I am not a microsoft advocate right now but I will see what windows 8 brings to the table.
Well getting back onto topic, PHP can be used to create anything from social networks as in facebook, myspace to search engines as in google but I would much rather use java and it's nio api to scrape sites to build an index but it really is such an amazing language of course there are those who would criticize it but it great for what it is.
$url = 'yousite.com';
what a simple language.